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Unformatted text preview: 20.462J/3.962J PS 4 solutions 1 of 5 9/4/06 Problem Set 4 Model Solutions 20.462J/3.962J Issued: 03/10/06 Spring 2006 Due: 03/17/06 100 points total 1. Physically associating polymers are used as drug carriers, tissue bulking agents, and cell delivery scaffolds. a. Explain why physical hydrogels are most often formed using block copolymers, where polymer chains are made up of repeating stretches of monomers which can take part in physical cross-links. Single physical cross-links, formed by hydrophobic association/Van der Waals bonding, ionic bonding, or hydrogen bonding between two monomers in water, have significantly lower strengths than single covalent bonds (10-100-fold lower bond energies). Thus, formation of even transiently stable hydrogels (or micelles) requires block copolymer structures where cooperative binding can occur between bonding blocks of two chains. Formation of one physical bond is immediately followed by bonding of several other adjacent monomer units, ‘zipping up’ a physical contact between two chains. b. (15 points) For the 3 types of gels described below, provide the following 3 pieces of information: (A) State what type of polymer functional groups would be used to create physical gels with the described properties, (B) give one specific example of such a functional group, showing how it forms physical crosslinks, and (C) state how that functional group or groups provide the desired behavior: i. Gels that are stable at reduced temperatures but dissolve at elevated temperature. ii. Gels that are stable at elevated temperatures but dissolve at reduced temperatures. iii. Gels that are stable in low ionic strength buffers but dissolve in high-ionic strength buffers....
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- Spring '03
- Polymer, Vdry, mM ionic strength, physical gels